The subject matter disclosed herein relates to an aircraft engine fuel system and, more particularly, to an aircraft engine fuel system including a pump unloading valve.
A conventional fuel supply system uses a positive displacement pump, for example a gear pump, which is driven by an engine through an accessory gearbox, to provide fuel through a fuel metering system to the burners of the engine. The positive displacement pump which, in the interests of clarity will be referred to herein as a “gear pump”, receives fuel from a fuel supply and has a rotational speed directly proportional to the shaft speed of the engine. Generally, the capacity and therefore the size of the pump is calculated on the basis of the maximum fuel flow which will be needed in use and a safety margin is applied on top of that maximum. Thus, there will be many operating conditions, notably engine idle conditions where the output of the pump exceeds the demand of the engine.
Excess fuel from the pump output is spilled back to the low pressure side of the fuel system to maintain a substantially constant pressure drop across the fuel metering valve of the fuel metering system. A pressure raising and shut-off valve (PRSOV) is interposed between the metering valve and the engine burners, and ensures that the fuel system upstream of the PRSOV is pressurized to a sufficient level that ancillary equipment powered by fuel pressure, for example engine control vanes, can be operated. Additionally, the PRSOV provides a means of isolating the gas turbine engine burners from the fuel supply system when the engine is to be shut-off.
The conventional system described briefly above often suffers from “heat rejection”. When the gear pump is pumping fuel against system pressure, the action of spilling fuel in excess of engine demand to the low pressure side of the system results in fuel heating. As fuel is used as a cooling medium for other engine system, the greater the level of rejection of heat into the fuel by the fuel pumps, the lower is the capacity of the fuel to cool other components. In addition, energy from the engine is wasted in heating the fuel through heat rejection at the fuel pumps thus resulting in an overall increase in engine fuel consumption.